Poisson Spot Research Articles

Mapping of near field light and fabrication of complex nanopatterns by diffraction lithography

We report a single-step lithographic approach for precisely mapping near field light diffraction in photoresist and fabricating complex subwavelength structures. This method relies on the diffraction of UV light from opaque patterns on a photomask, and utilizes the central diffraction maximum (known as the ‘Poisson spot’ for an opaque disk) and its higher orders. By correlating pattern geometries with the resulting diffraction, we demonstrate that the near field light intensity can be quantified to high precision and is in good agreement with theory. The method is further extended to capture higher order diffraction, which is utilized to fabricate unconventional subwavelength nanostructures with three-dimensional topographies. The simplicity of this process and its capability for light mapping suggest it to be an important tool for near field optical lithography.

Large-scale ordered silicon microtube arrays fabricated by Poisson spot lithography

A novel approach based on the Poisson spot effect in a conventional optical lithographysystem is presented for fabricating large-scale ordered ring patterns at low cost, in whichthe pattern geometries are tuned by controlling the exposure dose and deliberate design ofthe mask patterns. Following this by cryogenic deep etching, the ring patternsare transferred into Si substrates, resulting in various vertical tubular Si arraystructures. Microscopic analysis indicates that the as-fabricated Si microtubeshave smooth interior and exterior surfaces that are uniform in size, shape andwall-thickness, which exhibit potential applications as electronic, biological and medicaldevices.

Limits on achievable intensity reduction with an optical occulter

Deep shadowing of a normally incident plane wave by an opaque circular disk is partially negated by the formation of a region of strong intensity surrounding the axis passing normally through the disk center. This local intensity enhancement, historically referred to as the Poisson Spot (also known as the Spot of Arago), has been the principal source of difficulties in applications where a significant reduction of the incident intensity is essential. In particular, the NASA Terrestrial Planet Finder's (TPF) mission requires suppression of direct starlight by at least 10 orders of magnitude over the entire visible spectral range. One technique that has been proposed for blocking the direct starlight is to use a rotationally symmetric disk with petallike segments along its boundary. We find that, even though such configurations could, indeed, theoretically provide the desired intensity reduction, they would require unreasonably small radii of curvature at the petals' tips (in the range of micrometers). When the radii of curvature are increased to 3 mm, the intensity reduction drops to a modest 5 to 6 orders of magnitude. Given that for the NASA's TPF mission the proposed occulter radius would be on the order of 25 m, even the 3 mm radius of curvature would be too small for any practical implementation. Further increases of the radius of curvature result in progressively poorer intensity suppression. As an alternative solution we propose an apodized circular disk. We show that with an optimized apodization function, intensity reductions of at least 10 orders of magnitude can be achieved over the entire visible spectral range. Numerical results are presented for parameters appropriate to the NASA TPF mission.

Particle–wave discrimination in Poisson spot experiments

Matter–wave interferometry has been used extensively over the last few years to demonstrate the quantum-mechanical wave nature of increasingly larger and more massive particles. We have recently suggested the use of the historical Poisson spot setup to test the diffraction properties of larger objects. In this paper, we present the results of a classical particle van der Waals (vdW) force model for a Poisson spot experimental setup and compare these to Fresnel diffraction calculations with a vdW phase term. We include the effect of disc-edge roughness in both models. Calculations are performed with D2 and with C70 using realistic parameters. We find that the sensitivity of the on-axis interference/focus spot to disc-edge roughness is very different in the two cases. We conclude that by measuring the intensity on the optical axis as a function of disc-edge roughness, it can be determined whether the objects behave as de Broglie waves or classical particles. The scaling of the Poisson spot experiment to larger molecular masses is, however, not as favorable as in the case of near-field light-grating-based interferometers. Instead, we discuss the possibility of studying the Casimir–Polder potential using the Poisson spot setup.

New Prospects for de Broglie Interferometry

We consider various effects that are encountered in matter wave interference experiments with massive nanoparticles. The text-book example of far-field interference at a grating is compared with diffraction into the dark field behind an opaque aperture, commonly designated as Poisson’s spot or the spot of Arago. Our estimates indicate that both phenomena may still be observed in a mass range exceeding present-day experiments by at least two orders of magnitude. They both require, however, the development of sufficiently cold, intense and coherent cluster beams. While the observation of Poisson’s spot offers the advantage of non-dispersiveness and a simple distinction between classical and quantum fringes in the absence of particle wall interactions, van der Waals forces may severely limit the distinguishability between genuine quantum wave diffraction and classically explicable spots already for moderately polarizable objects and diffraction elements as thin as 100 nm.

High intensity x-ray diffraction in transmission mode employing an analog of Poisson’s spot

Poisson’s spot is a diffraction phenomenon producing an intensity maximum at the center of the geometric shadow of circular opaque objects. In an analog of the Poisson spot experiment, we show that a tubular cone of x-rays incident upon a crystalline sample produces diffraction spots or foci, corresponding to Bragg maxima within a transmission shadow. We discuss the beam geometry and the intensity gain recorded at the foci in transmission mode. We describe the geometric growth and decay of the foci over a linear axis with the aid of a movie sequence synchronized with the plotting of a diffractogram. The mean signal of a small central area in each successive camera image provides the intensity data for the diffractogram.

Easy demonstration of the Poisson spot

No abstract

Basic diffraction phenomena in time domain

Using a recently developed technique (SEA TADPOLE) for easily measuring the complete spatiotemporal electric field of light pulses with micrometer spatial and femtosecond temporal resolution, we directly demonstrate the formation of theo-called boundary diffraction wave and Arago's spot after an aperture, as well as the superluminal propagation of the spot. Our spatiotemporally resolved measurements beautifully confirm the time-domain treatment of diffraction. Also they prove very useful for modern physical optics, especially in micro- and meso-optics, and also significantly aid in the understanding of diffraction phenomena in general.

Poisson spot with magnetic levitation

In this paper we describe a unique method for obtaining the famous Poisson spot without adding obstacles to the light path, which could interfere with the effect. A Poisson spot is the interference effect from parallel rays of light diffracting around a solid spherical object, creating a bright spot in the center of the shadow.

Preliminary research on Poisson-line alignment technique

Poisson line technique is a method of generating a straight line by diffraction for high precision alignment，which can keep the center of Poisson spot in this line at long distances (several tens or hundreds of meters). Numerical simulation and experimental demonstration are proposed to research this technique. The intensity of the line increases asymptotically to the incident intensity as distance from the disk increases. The diameter of the line increases as the distances from the disk increases. Whereas it decreases as the diameter of the disks increases. The direction of the line is parallel to laser beam and propagates through the center of the disk，which is sensitive to center excursion and insensitive to the incline of the disk. Poisson line is sensitive to 10 μm excursion in simulation. And Poisson line technique can adjust 50 μm center excursion at long distance in experimental demonstration.

Objects in Telescope Are Farther Than They Appear

The wave nature of light is not part of students' common experiences, so often physics teachers and textbooks will add a historical anecdote about how scientists, too, were tricked by light. A common one is how, in the early 19th century, Poisson declared that since Fresnel's ideas on the wave nature of light implied that the shadow cast by a disk would contain a bright spot at its center, Fresnel's ideas were obviously flawed. The spot was later detected, proving Fresnel right! But recent studies of Galileo's work have brought to light a story about diffraction that may displace Poisson's spot as the favored historical anecdote, for it seems that diffraction tricked Galileo, too. Diffraction of light caused Galileo to mismeasure the distances to the stars.

First- and second-order Poisson spots

Although Thomas Young is generally given credit for being the first to provide evidence against Newton’s corpuscular theory of light, it was Augustin Fresnel who first stated the modern theory of diffraction. We review the history surrounding Fresnel’s 1818 paper and the role of the Poisson spot in the associated controversy. We next discuss the boundary-diffraction-wave approach to calculating diffraction effects and show how it can reduce the complexity of calculating diffraction patterns. We briefly discuss a generalization of this approach that reduces the dimensionality of integrals needed to calculate the complete diffraction pattern of any order diffraction effect. We repeat earlier demonstrations of the conventional Poisson spot and discuss an experimental setup for demonstrating an analogous phenomenon that we call a “second-order Poisson spot.” Several features of the diffraction pattern can be explained simply by considering the path lengths of singly and doubly bent paths and distinguishing between first- and second-order diffraction effects related to such paths, respectively.

Poisson’s spot with molecules

In the Poisson-spot experiment, waves emanating from a source are blocked by a circular obstacle. Due to their positive on-axis interference an image of the source (the Poisson spot) is observed within the geometrical shadow of the obstacle. In this paper we report the observation of Poisson's spot using a beam of neutral deuterium molecules. The wavelength independence and the weak constraints on angular alignment and position of the circular obstacle make Poisson's spot a promising candidate for applications ranging from the study of large molecule diffraction to patterning with molecules.

Compression of Arago Spot Images for Rapid Position Measurement of Inertial Fusion Energy Targets

A target position measurement method using a compressed Arago spot image is presented for real time data processing. To decrease the amount of data of the Arago spot image, the image is optically compressed into a one dimensional line image by a cylindrical lens. The experimental results for a 5 mm diameter target demonstrated a measurement accuracy of 0.35 μm when the target was 10 m from a charge coupled device (CCD) camera.

Application of target tracking method using an Arago spot and divergent beam for in-chamber measurement

A position measurement method using the Arago spot and a divergent beam is discussed for the in-chamber measurements of an inertial fusion energy (IFE) target. By employing a divergent beam, the actual displacement of the target is measured by the magnified displacement of the Arago spot, and the shadow of the target is also magnified, whereas the diameter of the Arago spot is not enlarged significantly. This fact enables us to measure the small displacement of the target with high measurement accuracy over a large measurement range. The Arago spot can be identified by the maximum intensity in the diffraction pattern due to the magnified target shadow. A measurement accuracy of 0.2 μm is achieved with a measurement range of 2 to 10 m for a 5-mm-diameter target.

Improving Visibility of Diffraction Pattern with Pseudo-Thermal Light

We report an experimental observation of Poisson's spot with pseudo-thermal light. The experimental results show that the diffraction pattern disappears in the intensity distribution behind the opaque disc but emerges through both auto-correlation and cross-correlation intensity measurements. The auto-correlation scheme can take care of both better visibility and higher resolution of the diffraction pattern under the condition that the thermal light source has a larger spectral bandwidth.

Propagation and diffraction of optical vortices

We explore the propagation and diffraction of optical vortices (Laguerre–Gaussian beams) of varying azimuthal index past a circular obstacle and Young’s double slits. When the beam and obstacle centers are aligned the famous spot of Arago, which arises for zero azimuthal index, is replaced for non-zero azimuthal indices by a dark spot of Arago, a simple consequence of the conserved phase singularity at the beam center. We explore how for larger azimuthal indices, as the beam and obstacle centers are progressively misaligned, the central dark spot breaks up into several dark spots of Arago. Using Young’s double slits we can easily measure the azimuthal index of the vortex beam, even for polychromatic vortices generated by broadband supercontinuum radiation.

Position Measurement Method Using a Divergent Laser Beam and Arago Spot for Tracking of an Inertial Fusion Energy Target

This paper is a complement to our previous work [Jpn. J. Appl. Phys. 46 (2007) 6000] on the accuracy of the position measurement method using an Arago spot for an inertial fusion energy target tracking system and we here report an improvement of the measurement accuracy using a divergent laser beam. By employing divergent beam illumination, we can magnify the displacement of the Arago spot compared with the actual displacement of the target without enlarging the diameter of the Arago spot significantly, allowing us to measure the small displacement of the target with high accuracy over a large measurement range. The experimental results for a 5-mm-diameter target demonstrated that a measurement accuracy of lower than 0.2 µm can be achieved when the distance between the target and a charge-coupled device camera is within the range from 2 to 10 m.

Target Tracking and Engagement for Inertial Fusion Energy - A Tabletop Demonstration

In the High Average Power Laser program, we have developed an integrated target tracking and engagement system designed to track an inertial fusion energy target traveling 50-100 m/s in three dimensions and to steer driver beams so as to engage it with ±20 μm accuracy. The system consists of separate axial and transverse detection techniques to pre-steer individual beamlet mirrors, and a final fine-correction technique using a short-pulse laser “glint” from the target itself.Transverse tracking of the target uses the Poisson spot diffraction phenomenon, which lies exactly on axis to the centroid of the target. The spot is imaged on a digital video camera and its centroid is calculated in ˜10 ms with 5 μm precision. In our tabletop demonstration, we have been able to continuously track a target falling at 5 m/s and provide a fast steering mirror with steering commands. We are on the verge of intercepting the target on-the-fly and of verifying the accuracy of engagement.Future work entails combining transverse tracking, axial tracking, triggering and the final “glint” system. We also will implement a verification technique that confirms successful target engagement with a simulated driver beam. Results and integration progress are reported.

A Continuous, In-Chamber Target Tracking and Engagement Approach for Laser Fusion

Target engagement is the process of measuring the target trajectory and directing the driver beams to hit the target at a position that is predicted based on these measurements. New target engagement concepts have been proposed in the last few years to continuously track the targets and to verify that the tracking system is aligned with the driver beams for each shot.For transverse position, a laser beam continuously backlights the target and the position of the Poisson spot in the center of the target’s shadow is measured. Axial target displacement is measured using a laser interferometer and counting interference fringes as the target moves away from the laser source. Final steering corrections use a “glint” reflected off the target ˜1 ms prior to firing the laser beams and collected in a separate Position Sensitive Detector (PSD) for each driver beamlet. The position of the glint on the PSD is compared to the position of an alignment beam that is collinear with the driver beam. Steering corrections are then made based on the difference in position of the two spots reaching the PSD.